1. Field of the Invention
The present invention relates to apparatus for extruding plastic materials, and more particularly, to apparatus for extruding uniform coverings of plastic insulating materials upon continuously advancing filamentary cores, which may be, for example, electrical conductor wires.
2. Technical Considerations and Prior Art
In the manufacture of communications wire and cable, insulating plastic coverings are extruded over a filamentary core, such as a wire or a cable core, by extrusion apparatus of the cross head type, which includes an extrusion cylinder having a stock screw rotatably mounted within a cylindrical bore therein for forcing an extrudable plastic compound from an entrance end of the bore to a delivery end thereof. The filamentary core is advanced through an extrusion head mounted at the delivery end of the bore and formed with an extrusion chamber or passage extending transversely across and communicating with the extrusion bore, for receiving a stream of plastic compound in a viscous fluent state. A forming die, concentric with and positioned in an exit end of the extrusion chamber, is positioned in axial alignment with and spaced from a core tube or guide positioned in the chamber, which passes and guides the advancing core through the axial center of an extrusion orifice in the die.
Extrusion apparatus of the foregoing type necessitates that the stream of fluent plastic material undergo at least one 90.degree. bend in its flow path, producing flow imbalance conditions in the plastic compound characterized by differences in flow rates and fluid pressures in those portions of the extrusion head proximate to and remote from the delivery end of the extrusion bore. These flow imbalances are a frequent cause of eccentricity, or lack of concentricity, between the plastic insulating sheath and the core of the extruded product and/or out-of-roundness of the outer circumference of the plastic insulating sheath. This results in cross-sectional and longitudinal variations in the wall thickness of the covering on the core. Such variations are virtually intolerable in insulated wire conductors utilized for electrical communications, since the electrical characteristics thereof must be substantially uniform. Therefore, the concentricity, roundness and uniformity of the insulating covering must be stringently controlled.
The foregoing inherent unbalanced flow conditions within a cross head type plastic insulation extruder are magnified when the plastic insulating material extruded onto the filamentary core is of a highly viscous or highly elastic type, for example, polypropylene.
Various methods and apparatuses have been devised in an attempt to solve the problem of flow imbalance in cross head extruders with varying degrees of success. These have included, among other things, dividing the stream of plastic material delivered to the chamber into a plurality of smaller streams which are subsequently recombined around the forward end of the core tube adjacent to the die, in an attempt to equalize the flow of plastic onto the core at circumferentially spaced points therearound. This was done on a volumetric or flow rate basis, neglecting the pressure and velocity distribution in the plastic streams, to attempt to obtain an equal quantity of plastic material in each stream combining around the filamentary core.
While many prior art flow balancing techniques have resulted in the production of an extruded product having improved concentricity of the insulating sheath relative to the core and improved roundness of the product, they have been found to be unsatisfactory when the plastic material utilized is of the highly viscous or highly elastic type. A stream of highly viscous fluid, such as molten polypropylene, when moving through a conduit or closed channel, is subjected to shear stresses which result in substantial velocity and pressure gradients, particularly in and subsequent to a curve or bend in the channel, before steady state fluid flow has been established in the stream. It is therefore imperative that these gradients be taken into consideration when the fluid stream in the channel is further divided into smaller streams in order to assure that the velocity and pressure gradients in each of the smaller streams is identical. Otherwise each smaller stream will not deliver an equal quantity of plastic to the core and the plastic covering formed thereon will not be uniform therearound and may be eccentric with respect thereto.